The present invention relates to a device to hold sheet form elements when they are inserted one by one into a work station fitted to a converting machine.
The invention finds a particularly advantageous, but not exclusive, application in the field of manufacturing cardboard packages.
It is known practice to cut blanks in a succession of sheets by means of a converting machine commonly known as a cutting press. In this type of machine, each sheet is inserted successively into a cutting station inside which the actual cutting operation takes place, then into a stripping station where the waste generated by the previous step is removed.
Each sheet is inserted individually in a precut form into such a waste stripping station. Specifically, the blanks are cut but are still attached to one another by attachment points. The same applies to many sheet portions that are of no final use and that are therefore considered to be waste.
When a cut sheet decelerates in the stripping station, before stopping between the ejection tools, its rear portion naturally tends to catch up with its front portion that is held by the gripper bar. This phenomenon is particularly noticeable when the sheets are relatively light in weight and/or large in size.
However that may occur, its consequence is that the flatness of the sheet is substantially deformed. This increases by the same amount as the risk of offset relative to the tools. It is known that the ejection operation requires precision in the prior positioning of the sheet. The precision naturally is all the finer if the waste has small dimensions.
In order to remedy this problem, thought has been given to holding each cut sheet during its insertion into the stripping station by holding it partially by its rear portion. For this, systems have notably been developed that generate a local suction on one face of the sheet. We are thinking notably in this instance of a Bernoulli tablet placed crosswise at the entrance to the stripping station.
This type of arrangement however has the drawback of providing insufficient effectiveness with sheets having a low basis weight, notably with those of less than 400 g. Specifically, when the sheet is too light, its rear portion tends to float during the movement and thus be relatively distant from the Bernoulli tablet. The latter can then not correctly fulfil its suction function.
In the end, this makes the positioning of the sheet more than approximate and this therefore generates inaccuracy at the time of ejection of the waste. But the lack of effectiveness of the Bernoulli tablet also causes banging at the rear portion of the sheet. The result of this is that many attachment points tend to break which becomes problematic at the time of ejection of the waste and often forces the operator of the converting machine to lower the production rate.